The present invention pertains in general to low impedance crossunders and in particular to low impedance crossunders within an active device.
Planar technology is most commonly used in the fabrication of integrated circuits. In this technology, the devices and components forming the integrated circuit extend below the surface of one plane of a silicon substrate. The devices and components are formed by selected diffusion of dopants into the substrate, each dopant creating a region of a characteristic conductivity type in the volume into which it diffuses. In order to control the zones of diffusion of the dopant, the layer of silicon to be treated is first covered by a layer of silicon dioxide, SiO.sub.2. The silicon dioxide is then coated with a photosensitive material known as the photoresist which may, for example, polymerize and harden in the presence of ultraviolet light. A photographic negative of a pattern of desired windows to be etched in the SiO.sub.2 layer is placed on the surface of the photoresist and ultraviolet light is shined through the negative to harden the photoresist except in those areas covered by the window pattern. After removing the negative and dissolving the unexposed portions of the photoresist, the SiO.sub.2 layer is etched away in those areas not covered by the hardened photoresist by exposure to a buffered solution of hydrofluoric acid in order to provide a pattern of exposed portions of the layer to be doped beneath the SiO.sub.2. A dopant tending to produce a region of a desired conductivity type is then diffused into the layer through the window. This process is repeated as often as is required to produce the desired number and pattern of regions of different conductivity types. Finally, the layer of SiO.sub.2 is formed on the external surface of the plane of the silicon substrate through which insulating layer preohmic windows are etched to delimit electrical contact portions. A metallization pattern is then deposited on the surface of the insulating SiO.sub.2 layer and through the preohmic windows to make electrical contact with the diffused regions.
It is common to have an integrated circuit in which at least some of the plurality of the conductors formed by the pattern of metallization must cross in order to form a desired connection between devices and components in the integrated circuit.
One approach to providing crossings for conductors on the surface of an integrated circuit involves the formation of a first region of a first conductivity type surrounded by a second region of a second conductivity type, both of which pass beneath the SiO.sub.2 layer which is in turn below the conductor to be crossed. A crossunder is formed which may be electrically connected to a metallization pattern on both sides of a conductor to be crossed. By electrically shorting the two regions, transistor action between the two layers is prevented. In this approach, crossunders are isolated from one another and from active devices being surrounded by a region of the first conductivity type. However, as integrated circuit dimensions have shrunk, less of the area of an IC chip may be practically devoted to crossunders of this type. Furthermore, with decreased IC dimensions a need has already arisen for providing a low impedance connection between contacts to an active device on both sides of a conductor passing over the active device.